A Multiple-Input Multiple-Output (hereinafter, referred to as MIMO) system refers to a system for improving data transmission and receive efficiency using multiple transmit antenna and multiple receive antenna. The MIMO technique includes spatial diversity and spatial multiplexing. The spatial diversity is a technique for improving transmission reliability by transmitting the same data through multiple transmit antenna. The spatial multiplexing is a technique for transmitting data in a high speed without increasing bandwidth of a system by transmitting different data through multiple transmit antenna. The spatial diversity and the spatial multiplexing may be combination.
Generally, a base station provides services to a plurality of user equipments in one or more cells. The base station schedules and transmits user data for the plurality of user equipments. At this time, the base station also schedules and transmits control information so that the user equipments may find and process the user data. A channel on which the user data is loaded is referred to as a data channel, and a channel on which the control information is loaded is referred to as a control channel. The user equipment decodes the user data on the data channel using the control information on the control channel.
In a MIMO system, control information includes information related to MIMO information and is transfers to to the user equipment on the control channel. The user equipment decodes the user data transmitted through multiple antenna using the control information. For example, the base station modulates the control information in Quadrature-Phase Shift Keying (QPSK), encodes the control information at a coding rate such as 2/3, and transmits the coded control information. The user equipment blind-detects and decodes the control information using the coding rate 2/3. The blind-detecting technique is a method of decoding given data until the user equipment finds its data when the user equipment does not know whether the data transmitted from the base station is its data. Considering the amount of operation, the blind-detecting technique is allowed when the number of information bits related to MIMO is fixed.
However, the MIMO technique may be modified in a variety of ways to be applied, and the number of information bits related to MIMO information may be diversely changed. So it may be inefficient if radio resources for the MIMO information occupies in amount of maximum number of information bits. For example, when user data is transmitted without applying codebook-based precoding, some of radio resources assigned for a precoding matrix index (PMI) which is one of the MIMO information can be useless.
A method is sought for efficiently transmitting the MIMO information.